package com.sweet.service;

/**
 * Created by sweet on 2018/11/26.
 * ---------------------------
 */
public final class IdUtils {
    private long workerId;
    private long datacenterId;
    private long sequence = 0L;
    private long twepoch = 1288834974657L;                              //  Thu, 04 Nov 2010 01:42:54 GMT
    private long workerIdBits = 5L;                                     //  节点ID长度
    private long datacenterIdBits = 5L;                                 //  数据中心ID长度
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);             //  最大支持机器节点数0~31，一共32个
    private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);     //  最大支持数据中心节点数0~31，一共32个
    private long sequenceBits = 12L;                                    //  序列号12位
    private long workerIdShift = sequenceBits;                          //  机器节点左移12位
    private long datacenterIdShift = sequenceBits + workerIdBits;       //  数据中心节点左移17位
    private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits; //  时间毫秒数左移22位
    private long sequenceMask = -1L ^ (-1L << sequenceBits);                          //  4095
    private long lastTimestamp = -1L;

    private static class IdGenHolder {
      private static final IdUtils instance = new IdUtils();
    }

    public static IdUtils get(){
      return IdGenHolder.instance;
    }

    public IdUtils() {
      this(0L, 0L);
    }

    public IdUtils(long workerId, long datacenterId) {
      if (workerId > maxWorkerId || workerId < 0) {
        throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
      }
      if (datacenterId > maxDatacenterId || datacenterId < 0) {
        throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
      }
      this.workerId = workerId;
      this.datacenterId = datacenterId;
    }

    public synchronized long nextId() {
      //获取当前毫秒数
      long timestamp = timeGen();
      //如果服务器时间有问题(时钟后退) 报错。
      if (timestamp < lastTimestamp) {
        throw new RuntimeException(String.format(
            "Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
      }
      //如果上次生成时间和当前时间相同,在同一毫秒内
      if (lastTimestamp == timestamp) {
        //sequence自增，因为sequence只有12bit，所以和sequenceMask相与一下，去掉高位
        sequence = (sequence + 1) & sequenceMask;
        //判断是否溢出,也就是每毫秒内超过4095，当为4096时，与sequenceMask相与，sequence就等于0
        if (sequence == 0) {
          //自旋等待到下一毫秒
          timestamp = tilNextMillis(lastTimestamp);
        }
      } else {
        //如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加
        sequence = 0L;
      }
      lastTimestamp = timestamp;
      // 最后按照规则拼出ID。
      // 000000000000000000000000000000000000000000  00000            00000       000000000000
      // time                                       datacenterId   workerId    sequence
      return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift)
          | (workerId << workerIdShift) | sequence;
    }

    protected long tilNextMillis(long lastTimestamp) {
      long timestamp = timeGen();
      while (timestamp <= lastTimestamp) {
        timestamp = timeGen();
      }
      return timestamp;
    }

    protected long timeGen() {
      return System.currentTimeMillis();
    }

//  public static void main(String[] args) {
//    System.out.println(IdUtils.get().nextId());
//    System.out.println(IdUtils.get().nextId());
//    long x = IdUtils.get().nextId();
//    System.out.println(x);
//    System.out.println((x + "").length());
//    System.out.println(System.currentTimeMillis());
//  }
}
